1. Field of the Invention
The present invention relates to an image reading apparatus for providing electric signals corresponding to the image on an original by optical scanning thereon.
2. Description of the Prior Art
In an image reading apparatus employed, for example, in a facsimile apparatus of the like, an original moving in a determined direction is illuminated with light, and the reflected or transmitted light is focused through a lens onto a photoelectric converter, such as a photosensor array, to provide time-sequential electric image signals corresponding to a scanning line. Said photosensor array is usually composed of a charge-coupled device array or a photodiode array on which the image of a scanning line is focused by said lens. In the facsimile apparatus, the electric image signals are converted into suitable codes and transmitted to a distant place for example through a telephone line. Upon completion of reading of a scanning line, an original feed mechanism is activated to advance the original by a scanning pitch, and to conduct the reading of the next scanning line. The transmission of the original image is achieved by repeating the above-mentioned procedure.
In this case the resolving power of the image is determined by that of the photosensor array, which is equal to the number photoelectric converting elements in said photosensor array. Usually the number of said elements in a photosensor array is limited to 1500-2000. It is necessry to increase the number of said elements for example when the original is a microfilm, but a photosensor array including 3000-4000 elements is expensive. It may also be possible to project an enlarged image onto plural photosensor arrays arranged linearly, but the signals are inevitably lost between the neighboring photosensor arrays where the photosensitive elements are not present. For this reason there have been employed plural sets of an imaging lens and a photosensor array, but such a solution is defective in requiring plural units of expensive lenses.